This invention generally relates to brazing methods for use with superalloys. More particularly, this invention relates to a brazing process employing a braze tape to inhibit oxidation of the faying surfaces of components being joined during a brazing process.
Nickel, cobalt, and iron-base superalloys are widely used to form high temperature components of gas turbine engines. While some high-temperature superalloy components can be formed by a single casting, others are preferably or required to be fabricated by a joining operation. As an example, a high pressure turbine nozzle assembly may be structurally supported by a nozzle support assembly formed by brazing a number of individual nickel-base superalloy members. In carrying out the brazing process, an appropriate braze alloy is placed between the interface (faying) surfaces to be joined, and the faying surfaces and the braze alloy therebetween are heated to a temperature sufficient to melt the braze alloy without melting or causing grain growth in the superalloy base material. The braze alloy melts at a lower temperature than the superalloy base material as a result of containing a melting point suppressant such as boron. On cooling, the braze alloy solidifies to form a permanent metallurgical bond.
Because gas turbine engine components must operate in a thermally hostile environment that requires resistance to oxidation, superalloys typically contain aluminum, titanium, iron, and/or niobium that, in addition to contributing to the mechanical properties, form tenacious oxides that inhibit oxidation of the superalloy. Unfortunately, if a superalloy contains a sufficient amount of these metals individually or in combination (e.g., by weight, more than 0.4% aluminum, more than 0.7% titanium, or more than 0.7% Ti+Al), these protective oxides can hinder brazing of the superalloy by preventing the braze alloy from adequately wetting the surface of the superalloy. Because the braze alloy does not adequately flow onto the superalloy base material, uniform and complete brazing is not achieved.
As a solution, the faying surfaces of nickel-base superalloy components have been plated with nickel prior to the brazing operation. The nickel plating improves the brazeability of a superalloy by preventing oxide formation in those regions that are to be bonded, thereby enhancing the wetting action of the braze alloy. Such a process typically involves masking a superalloy component to expose only those surfaces to be plated, and then depositing a layer of nickel using an electrochemical plating technique. However, nickel electroplating processes have several shortcomings, including being costly, labor intensive, time consuming, and environmentally unfriendly. Accordingly, it would be desirable if a brazing process existed that facilitated the brazing of superalloy components without the need for nickel plating the surfaces to be brazed.